Who is Vannier from The High Window and what is their importance?

Louis Vannier is a wealthy playboy, who is mixed up in various criminal schemes. Vannier is blackmailing Mrs. Murdock, having an affair with Lois Morny, and the mastermind behind the plot to counterfeit a valuable collectible gold coin owned by Mrs. Murdock. Louis is responsible for killing Phillips and Morningstar. In the end, he is killed by Leslie Murdock when he refuses to hand over the evidence against her mother.